Abstract

A hybrid zone occurs where two distinct genetic forms meet, mate and produce offspring with mixed genomes. Such zones may
vary in width, length and patchiness, and are found between species, subspecies, races or forms. Stable hybrid zones may be
maintained by selection against hybrids, environmental selection, or a combination of the two. A hybrid zone can arise either
by direct environmental selection in contiguous populations or by renewed contact between previously isolated populations.
Hybrid zones act as semi‐permeable barriers, which allow gene exchange for neutral or adaptive characters, whereas restricting
introgression of alleles that contribute to local adaptation or reduced hybrid fitness. The study of genomic regions that
experience barriers to gene flow can provide an important window for identifying specific genes and mutations that underlie
reproductive isolation and local adaptation. With the help of recent technological advances in development of thousands of
molecular markers, distributed genome‐wide, identification of such genomic regions is becoming possible in natural hybrid
zones.

Key Concepts:

A hybrid zone is a narrow geographic region where two genetically distinct populations or species are found in close proximity
and hybridise to produce offspring of mixed ancestry.

Hybrid zones are widespread, both geographically and across animal and plant taxa.

A hybrid zone is maintained by a balance between selection and dispersal.

The hybrid zone in M. m. musculus and M. m. domesticus. (a) The solid purple line indicates the location of the hybrid zone. An approximate position of the Norwegian/Swedish zone
is shown as a dashed line. Six extensively studied transects are shown as yellow rectangles. (b) The detailed location of
the hybrid zone at the Czech transect (D). Red circles indicate 228 sampling sites. The yellow dashed line depicts the course
of the zone based on consensus of six autosomal and nine X‐linked loci. Map by courtesy of Miloš Macholán.

Figure 2.

Effects of environment on hybrid zone structure and position. Two genotypes strongly adapted to two distinct habitats with
sample transects taken across (a) a linear zone and (b) a mosaic zone. (c, d) The respective character clines resulting from
these transects. Note differences in width and shape. (e) The distribution of individuals through a low‐density region, where
the zone is not at the density trough and (f) the dispersal of parental types (arrows) to bring the hybrid zone to rest in
the centre of the density trough.

Figure 3.

Shape of sigmoid and stepped clines (blue and red lines, respectively). Widths of each cline (w1 and w2) are measured as the inverse of the maximum slope (dashed lines) at the centre of the zone (c) (i.e. w1>w2). A sigmoid cline consists only of a hyperbolic tangent function (tanh), whereas a stepped cline consists of a steep central
region, defined by a tanh function, and shallow tails of introgression, defined by an exponential function. Note that the
extent of introgression at the left and right tails can be similar to (symmetric stepped cline) or different from each other
(asymmetric stepped cline).

Figure 4.

Phylogeography of European hedgehogs. (a) The ranges of Erinaceus europeus (west) and Erinaceus concolor (east), with (b) an mtDNA‐cytb phylogeny superimposed. Note the deep divergence (12%) between the nominate species, and their
cryptic subdivision into further distinct (6%) clades. This indicates separate survival in Mediterranean refugia over several
ice ages.

Butlin RK
(1998)
What do hybrid zones in general, and the Chorthippus parallelus zone in particular, tell us about speciation?
In: Howard DJ and
Berlocher SH (eds)
Endless Forms: Species and Speciation,
pp. 367–377.
New York: Oxford University Press.

Keller I,
Veltsos P and
Nichols RA
(2008)
The frequency of rDNA variants within individuals provides evidence of population history and gene flow across a grasshopper hybrid zone.
Evolution
62:
833–844.